Aspirator Hose

WARNING: This page is about a different car, the 2013 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.

Turn the ignition switch to ON, and press the A/C switch.
Turn the temperature control dial to maximum hot position.
Set the blow vents to the DEF position.
Turn the fan switch to "MAX" position.
Put a strip of paper close to the front side of in-vehicle sensor suction port (A) located in the instrument panel lower cover, and check that air is being sucked into the port by seeing the paper moving towards the port.
NOTE: Be careful not to let the paper get sucked into the port.

Fig 1: Identifying In-Vehicle Sensor Suction Port
Courtesy of SUBARU OF AMERICA, INC.
If the paper does not move at all, remove the instrument panel lower cover < Refer to REMOVAL , Instrument Panel Lower Cover. > and check for poor connection of the aspirator hose (A), in-vehicle sensor and heater unit, and repair them if necessary.

Fig 2: Checking For Poor Connection Of Aspirator Hose
Courtesy of SUBARU OF AMERICA, INC.